At start of electric discharge machining, positioning of a workpiece and a tool electrode must be made. Conventionally, short-circuit detecting means is employed for detection of the workpiece being brought at its reference surface in contact with the tool electrode, while the workpiece and the tool electrode are moved relatively to each other. The thus detected contact position is used as a basis for the positioning. More specifically, a typical short-circuit detecting means comprises a comparator having first and second input terminals which are supplied with a short-circuit detecting voltage and a reference voltage, respectively. The comparator is so arranged as to generate a signal, indicative of the workpiece being brought in contact with the tool electrode, in response to a change in the voltage appearing at the first input terminal connected to the tool electrode through a diode for shutting an electric discharge machining voltage, which change occurs when the workpiece and the tool electrode are brought in contact with each other so that the tool electrode is grounded through the workpiece.
In an electric discharge machine equipped with a short-circuit detecting means of the aforementioned type, the short-circuit detecting voltage is always applied, and hence the same voltage is applied in between the workpiece and the tool electrode through the diode when the electric discharge machining voltage is turned off. As a result, corrosion is liable to occur on the workpiece, in particular, of an electrolytically corrosive type.